The invention relates to a process and to a device for the separation of an aqueous emulsion containing oil.
A large amount of cooling lubricants and wash water are produced in the metalworking industry. These aqueous emulsions containing oil are relatively stable. This is because the relatively severe conditions to which they are exposed place relatively great demands on them in terms of stability. Because of their relatively high oil content, these types of aqueous emulsions containing oil should not be discharged to the drains. They therefore require special treatment before they are disposed of.
Known processes for the separation of emulsions include chemical processes, for example column separation, thermal processes, for example evaporation and combustion, as well as electrochemical processes. Examples of mechanical and physical processes including centrifuging and adsorption.
Owing to the high stability of aqueous emulsions containing oil, relatively simple mechanical and physical segregation processes are unsuitable. Emulsion separation using acids followed by neutralization leads to a relatively heavy salt load in the waste water. Electrochemical processes produce large volumes of sludge which in turn need to be processed.
DE 40 28 904 C1 describes a process for emulsion separation in which carbon dioxide is used. In this process, an emulsion which, apart from auxiliaries, essentially comprises oil, water and also an anionic emulsifier or an emulsifier combination of anionic and non-ionic surfactants, is either saturated with carbon dioxide under pressure or mixed with water containing carbon dioxide and, by subsequent slow heating, is separated into an organic phase and an aqueous phase. The separated emulsion is then discharged into a tank, where the water phase and oil phase separate. The process is preferably combined with a membrane filtration.
The relatively mild oxygen treatment using carbon dioxide leads to aqueous fractions whose purity is often not yet high enough to allow discharge to the drains.
The object of the invention is therefore to provide a process and a device for the separation of relatively difficult to separate aqueous emulsions containing oil, with which the disadvantages of the prior art are overcome and which produce an aqueous fraction that meets relatively stringent requirements in terms of purity.
The object is achieved by a process in which the aqueous emulsion containing oil is subjected for a specified time to an elevated pressure and an elevated temperature, mixed with carbon dioxide and subsequently relieved. The oil-depleted emulsion preclarified in this way is subsequently segregated in a separating tank. The resultant aqueous fraction is then separated by ultrafiltration into a retentate and water. The separation is carried out in an autoclave. A rising stream of carbon dioxide bubbles is produced by sudden pressure relief and an oil-rich phase is removed by the rising stream of carbon dioxide bubbles.
The essential advantage of the process according to the invention consists in that, through sudden pressure relief of the solution that is saturated with carbon dioxide, simultaneous release of gas bubbles is brought about throughout the entire emulsion. In this case, the separated oil fractions are removed by the upward motion of the gas bubbles in a first preclarification step through relief flotation, which brings about a substantial acceleration of the phase segregation. With the subsequent ultrafiltration, the aqueous phase that has segregated is cleaned to an extent such that this aqueous phase is sure to meet the purity requirements for discharge to the drains.
The invention provides that the aqueous emulsion containing oil is subjected to a pressure of up to 80 bar and subsequently relieved to approximately atmospheric pressure. This has the advantage that a relatively large amount of carbon dioxide is dissolved in the emulsion and dissociated to form carbonic acid. This leads to a medium strength acid. The relatively large amount of carbon dioxide that is degassed leads to good flotation.
Preferably, the aqueous emulsion containing oil is subjected to a pressure of from 3 to 10 bar. This leads to the advantage that the outlay on equipment is relatively low, and so only relatively low production costs are incurred.
According to the invention, the aqueous emulsion containing oil is subjected to a temperature of from 20xc2x0 C. to 60xc2x0 C. Such a temperature is advantageous because the pH and therefore the acid separation are improved.
It is proposed according to the invention that the aqueous emulsion containing oil is kept at the elevated pressure and the elevated temperature for a period from 5 to 20 min. As a result, advantageously, the maximum possible equilibrium concentration (pH) is achieved and the carbon dioxide is utilized particularly effectively.
According to the invention, after the carbon dioxide has been added, the aqueous emulsion containing oil has a pH lying in the range of between pH 3 to pH 7. A pH between pH 3 and pH 7 is advantageous since a pH of less than 7 is needed to carry out acid separation. The limit for carbonic acid lies at a pH of 3.
According to the invention, the retentate is fed back to the autoclave through a storage container after the ultrafiltration. This procedure has the advantage that there is no need to dispose of concentrated oil/aqueous solution mixtures, as is customary with ultrafiltration.
The object is also achieved by a device for carrying out the process according to the invention, which has an autoclave, a separating tank and an ultrafiltration system, an injection device for injecting fine bubbles of carbon dioxide being arranged in the bottom region of the autoclave.
Preferably, the injection device has a sintered baseplate, a bed of balls or one or more fine-meshed screening bases. The term xe2x80x9cfine-meshed screen basesxe2x80x9d is in this case intended to mean screening bases with apertures of a diameter in a range from 0.1 to 1 mm. The sintered baseplate preferably has apertures of a diameter in a range from 0.01 to 0.5 mm. The diameter of the apertures in the bed of balls is preferably in a range from 0.5 to 1 mm. The use of the sintered baseplate has the advantage that it results in a relatively high degree of uniformity for the injection. The use of a bed of balls is advantageous since, in this case, there is scarcely any risk of blockage and/or the balls are easy to clean. Use of one or, in particular, more fine-meshed screening bases is advantageous since they are easy to take out in order to be cleaned or can easily be replaced.